CC-1065 is a highly cytotoxic anti-tumor antibiotic isolated from cultures of Streptomyces zelensis. The CC-1065 molecule consists of three substituted pyrroloindole subunits linked by amide bonds. The “A” subunit is the alkylating cyclopropapyrroloindole (CPI) moiety, while the “B” and “C” subunits are identical pyrroloindole moieties.

Novel cytotoxic agent-cell binding agent conjugates comprising a cell-binding agent chemically linked to analogs of CC-1065 have been described [U.S. Pat. Nos. 5,475,092; 5,585,499; 5,846,545, R. V. J. Chari et al., Cancer Res., 55, 4079-4084 (1995)]. These cytotoxic agent-cell binding agent conjugates have therapeutic use because they deliver the cytotoxic agent to a specific cell population in a targeted fashion. In these cytotoxic agents, herein called DC1 and its derivatives, the alkylating CPI subunit “A” was replaced by the benzannelated analog cyclopropabenzindole (CBI).
The CBI unit can exist in the ring-closed cyclopropyl form or in the ring-open seco (chloromethyl) form. The “B” and “C” subunits were replaced with indole units. In addition, the terminal indole unit bears a substituent that allowed for linkage to cell-binding agents.

CBI is the precursor required for the synthesis of DC1 drugs and its derivatives. The original synthesis of CBI was described by D. L. Boger et al., [J. Org. Chem., 55, 5823-5833 (1990)]. An “improved” synthesis, also described by D. L. Boger et al., [J. Org. Chem., 57, 2873-2876 (1992)] is a 15-step process starting from naphthalene diol. Other pathways for the syntheses of CBI from different starting materials have also been described [K. J. Drost & M. P. Cava, J. Org. Chem., 56, 2240-2244 (1991), P. A. Aristoff & P. D. Johnson, J. Org. Chem., 57, 6234-6239 (1992)]. These syntheses are lengthy, time-consuming, expensive and provide poor yields.
A key step in the synthesis of CBI is the resolution of the enantiomers at the seco-CBI stage. Only the seco(−)enantiomer is biologically active, and it is important to efficiently remove the inactive (+) isomer. Isomer separation can be achieved, for example, by chiral HPLC. This method is not very efficient when applied to seco-CBI because the separation between the two enantiomers is poor. In addition, even the optimized separation on a chiral column is poor (retention time difference between the two isomers is less than 5 minutes), and requires a very non-polar solvent system, such as a mixture of 95% hexane and 5% isopropanol (Boger et al., 116, J. Am. Chem Soc., 7996-8006 (1994). Under these conditions, seco-CBI is poorly soluble, resulting in low efficiency (small loading amounts) on the column, and thus, long processing times. Alternatively, the enantiomeric mixture can be converted into a set of diastereomers by esterification with a chiral acid, such as mandelic acid, followed by separation by HPLC. However, the separated ester has to be hydrolyzed and then repurified, thus adding an extra processing step.
The therapeutic utility and promise of drugs such as DC1 and its derivatives, for example in the treatment of various cancers, makes it desirable that improved synthetic methods be developed in order to be able to manufacture CBI in large scale, by a simple, easily scalable, high-yield, inexpensive process that uses inexpensive and easily available starting materials.
The present invention provides such an improved synthetic method that addresses the aforementioned shortcomings of the prior art. All these advantages and more are provided by the invention described herein, as will be apparent to one of skill in the art upon reading the following disclosure and examples.